Various methods and apparatus have been proposed to allow user interaction with projected content. For example, U.S. Pat. No. 8,497,841 “SYSTEM AND METHOD FOR A VIRTUAL KEYBOARD” issued to Sze et al. (hereafter referred to as “Sze”) discloses an infrared (IR) light plane projected just above and parallel to a projection surface. The apparatus disclosed by Sze detects objects when they break the IR light plane, so they are only detected when very near that plane. Mid-air gestures are not recognized because they are above the IR light plane. Furthermore, the apparatus of Sze is sensitive to anomalies in the projection surface that could break or distort the IR light plane.
Optical time-of-flight (ToF) measurement is a proven method of range sensing that works by measuring the flight time of an emitted light signal which reflects off a point in the field of view. Pulses of light can be scanned out in a two dimensional pattern and collected into a point cloud that represents distance to objects in the scanned field. See e.g., US 2013/0107000 “SCANNING LASER TIME OF FLIGHT 3D IMAGING” by Xue et al.
ToF provides an appealingly compact solution for medium-long range depth imaging, but is limited in resolution at close distances due to limitations in the electronic circuitry to accurately and inexpensively measure short time scales and compensate for changes in the returned pulse amplitude.